1. Field of the Invention
The present invention relates to a sound quality enhancement circuit for audio signals and an audio amplifier circuit using the same and more specifically, relates to a sound quality enhancement circuit which enhances sound clarity, easily realizes an upper position of sound image for sound image positionality, is easily formed in an IC and is suitable for driving, in particular, a small full range corn type speaker.
2. Background Art
Different from an acoustic space where a full scale component stereo is disposed, an acoustic space where an audio apparatus including a car stereo and such as a TV apparatus (such as a TV receiver and a TV video game machine), a DVD, a mini component stereo and a computer audio is disposed is frequently either comparatively limited because such as a variety of apparatuses and furniture are disposed therein or comparatively narrow such as a room in a car. Moreover, since the speaker used is a small size, even in a case of a full range corn type dynamic speaker (herein below called as corn type speaker) or a two way speaker, the crossover frequency is high, for example, more than 5000 Hz, therefore a speaker system constituted primarily by a full range speaker combining a substantially a full range corn type speaker and a tweeter is frequently used.
Now, as important factors required for a speaker with regard to sound reproducibility, three characteristics, in that sound pressure frequency characteristic, amplitude characteristic and transient characteristic are enumerated. Among these, a speaker transient characteristic is recently drawn attention. In case of a corn type speaker, an offset of rise up vibration characteristic with respect to the speaker input signal is caused depending on frequencies. Such offset increases in a low frequency region (audio signal of frequencies less than 200 Hz) and gradually decreases toward a high frequency region (audio signal of frequencies more than 4.0 kHz) and the delay thereof also decreases.
The reason of the above is that although a corn type speaker produces a sound through vibration of the corn driven by a voice coil motor, since components in the low frequency region are produced through vibration of the entire corn and components in the high frequency region are radiated from a center portion of the corn (dome portion), the rise up speed of the high frequency components is high and the rise up speed of the low frequency components is delayed because of inertia difference of the vibrating corn. The offset increases depending on the size reduction of the small speaker.
On one hand, when a speaker is driven by applying a power output signal representing an audio signal to the speaker through such as equalizer and a filter and even if the power output signal shows a flat frequency characteristic, if there is a delay in the high frequency region with respect to the low frequency region, a delay is generally caused in the harmonic components of the audio signal and the clarity of the sound obtained from the speaker is deteriorated. In order to restore the deteriorated clarity, the high frequency region is generally boosted, however, the sound becomes hard which is problematic.
For this reason, with regard to a small size speaker, a predetermined phase characteristic is added by correcting the phase of the input signal, and further the sound clarity is enhanced by increasing the amplification degree in the high frequency region.
With regard to the above referred to acoustic apparatuses which are used in a comparatively narrow space, in order to enhance the sound image and clarity of the sound in view of transient characteristic of the speaker, a technology is also known which takes into account of head related transfer function (HRTF) representing acoustics transferred to a human, shifts a sound image produced from a lower position toward an upper position for sound image positionality and increases the sound clarity by making use of a sense of human with respect to sound in vertical direction.
Further, it is known that the frequency components relating to the sense in the upper position of sound image for sound image positionality are ones near 4 kHz and 8 kHz. The sound image varies upward by emphasizing the frequency components near to 4 kHz and 8 kHz. It is likely known that the frequency components that contribute clarity of human voice are near at 2.5 kHz.
BBE process technology that is one of sound quality enhancement technologies performs a correction process for audio signals for increasing the clarity thereof, in which the harmonic components delayed with respect to the fundamental wave are moved before the fundamental wave to approximate the sound wave form to natural sound and further the high frequency components are boosted. The sound quality enhancement thereby achieves a large effect with respect to the clarity through synergistic effect of phase correction and high frequency region boosting with a less boosting amount compared when an equalizer is used. Further, an example of BBE process technologies is disclosed in JPH10-75138A. Still further, the same type of sound quality enhancement circuit is disclosed in JP Patent No. 2,609,065.
In the BBE process technology as disclosed in JPH10-75138A, the audio signals are divided into high frequency region, intermediate frequency region and low frequency region, the signals are respectively processed by preamplifiers of three channels, after providing delays for the signals through filtering process and adjusting the phase of the audio signals, the signals processed through the respective channels are synthesized by mixing. Thereby, the clarity of the sound generated from the speaker is enhanced. This technology is utilized such as in recording studios and broadcasting stations.
Specifically, for the low frequency region, audio signals having frequencies less than 150 Hz are dealt, for the intermediate frequency region, audio signals having frequencies in a range of 150 Hz˜1.2 kHz are dealt and for the high frequency region, audio signals having frequency more than 1.2 kHz are dealt. An example of gain and phase correction characteristic by the BBE process technology is shown in FIG. 5 (a) and (b).
As seen from FIG. 5 (b) in which phase characteristic with respect to frequencies is shown, the phase is gradually delayed along a linear inclination depending on logarithmic increase of frequencies from the low frequency region to the intermediate frequency region and to high frequency region and resultantly, a characteristic delaying the phase by 360 degree over the range of audible frequencies (20 Hz˜20 kHz) is provided.
In the gain characteristic as shown in FIG. 5 (a), a crest is provided near at 600 Hz and at the both sides thereof, for example, in a range of 600 Hz˜3 kHz the amplitude is enhanced by about 10 dB/dec. Thereby, a characteristic which enhances the frequency components near and above 2.5 kHz is obtained.
However, in the above sound quality enhancement processing, since the frequency region has to be divided into three regions, when the signals in the three regions are synthesized, a slack is caused in the intermediate frequency region and a problem of sound drop off is generated.
On the other hand, in the gain and phase correction characteristic by means of the technology in which a position of sound image for sound image positionality is shifted toward an upper in vertical direction by taking into account the head related transfer function, as seen from FIG. 6 in which phase and gain characteristic with respect to frequencies is shown, the phase variation in the phase characteristic is not so significant, a characteristic having a crest near at 500 Hz is provided. With regard to the gain characteristic, the boosting is started near at 300 Hz, the amplitude in the range of 300 Hz˜1 kHz is enhanced by 9 dB/dec and the gain characteristic from the low frequency region to the intermediate region is improved. Thereby, the frequency components of 2.5 kHz and above are emphasized, clarity of sound is enhanced and the sound image is shifted upward.
On one hand, in a small full range corn type speaker or a system constituted substantially by a small full range speaker, since the diameter of the voice coil is shorten and the efficiency thereof is reduced, the sound clarity can not be increased, even if the BBE process technology is applied. Moreover, with regard to the gain characteristic, if the boosting is started near at 300 Hz, the amplitude in the range of 300 Hz˜1 kHz is enhanced by 9 dB/dec and the gain characteristic is improved, the sound quality in voice region of 500 Hz˜1 kHz varies and a problem of increasing noises arises.
In particular, in connection with cell phones using a small speaker having diameter of 1.5 cm˜3 cm, it is difficult to expect sound quality enhancement with the above technology.